Poly(arylene sulfides) (hereinafter abbreviated as “PASs”) represented by poly(phenylene sulfide) (hereinafter abbreviated as “PPS”) are engineering plastics excellent in heat resistance, chemical resistance, flame retardancy, mechanical properties, electrical properties, dimensional stability and the like. The PASs are commonly used in a wide variety of fields such as electrical and electronic equipments and automotive equipments because they can be formed or molded into various kinds of molded or formed products, films, sheets, fibers, etc. by general melt processing techniques such as extrusion, injection molding and compression molding.
As a typical production process of a PAS, is known a process, in which an alkali metal sulfide that is a sulfur source is reacted with a dihalo-aromatic compound in an organic amide solvent such as N-methyl-2-pyrrolidone. As the sulfur source, a combination of an alkali metal hydrosulfide and an alkali metal hydroxide is also used.
Secondarily produced salts such as NaCl, oligomers, polymerization aids, decomposition products and/or the like are contained together with a PAS formed in the reaction mixture by the polymerization reaction of the sulfur source with the dihalo-aromatic compound in the organic amide solvent. It is difficult to sufficiently remove these impurities even when the granular polymer formed is separated from the reaction mixture by sieving and washed with water. In many cases, a step of washing the PAS formed with the same organic amide solvent as the polymerization solvent, or an organic solvent such as a ketone (for example, acetone) or an alcohol (for example, methanol) is thus arranged as a post treatment step. Among these organic solvents (hereinafter also referred to as “washing solvents”) for washing, acetone and methanol are commonly used because they are easily recovered by distillation owing to their low boiling points, and are excellent in detergency.
These washing solvents are repeatedly used in the washing step of the production of PASs. When a washing solvent is repeatedly used, a liquid component such as a filtrate containing many impurities, an organic amide solvent, etc. in addition to the washing solvent is distilled after the washing step to recover the washing solvent.
However, the present inventors have found that when a washing solvent such as acetone is repeatedly used (i.e., “recycled through”) in the washing step, the mere purification of the washing solvent by distillation does not permit sufficiently removing a minute amount of impurities that adversely affect the quality of a PAS formed, and the physical properties of the PAS after washing are adversely affected.
More specifically, it has been proven that when the washing solvent is repeatedly recovered to reuse it in the washing step, the PAS after the washing is colored, or the melt viscosity of the PAS is lowered in an extreme case. As a result, the washing solvent cannot be used further repeatedly.
Further, although a PAS formed is treated with an aqueous solution or organic solvent solution of an acid or a salt (for example, ammonium chloride) of a weak alkali and a strong acid in a post treatment step in order to raise the crystallization temperature (Tmc: also referred to as “melt crystallization temperature”) of the PAS, the effect by the acid or salt is reduced when the washing solvent is repeatedly recovered by distillation to recycle it through the washing step, so that the crystallization temperature is hard to be raised. When the crystallization temperature of the PAS is lowered, the efficiency of a molding operation is reduced because, for example, the cycle of injection molding is elongated.
However, recognition as to purification of the washing solvent used in the washing step of the PAS has heretofore been too low to propose effective means for solving such problems as described above. On the other hand, some methods have been proposed as to purification of the organic amide solvent used in the polymerization step.
As a process for recovering N-methyl-2-pyrrolidone from a slurry of a PAS obtained by reacting an alkali metal sulfide with a dihalo-aromatic compound in N-methyl-2-pyrrolidone to purify it, there has heretofore been proposed a process comprising adding a small amount of an alkali metal hydroxide and/or an alkali metal carboxylate to a liquid comprising N-methyl-2-pyrrolidone as a principal component, and then conducting distillation (for example, Japanese Patent Application Laid-Open No. 11-349566).
As a process for recovering N-methyl-2-pyrrolidone from a slurry of a PAS obtained by reacting an alkali metal sulfide with a dihalo-aromatic compound in N-methyl-2-pyrrolidone to purify it, there has also been proposed a process comprising adding a small amount of ammonia or amine to a liquid comprising N-methyl-2-pyrrolidone as a principal component, and then conducting distillation (for example, Japanese Patent Application Laid-Open No. 2001-354769).
However, the processes for recovering and purifying the organic amide solvent by adding such alkaline compound(s) are not suitable for recovering and purifying the washing solvents such as acetone and methanol. In fact, even when any of the above-described processes is applied to a liquid comprising such a washing solvent as a principal component after the washing step, lowering of the crystallization temperature of a PAS formed or coloring thereof cannot be prevented when the washing solvent recovered is reused in the washing step.